The present invention relates to the signal processing art. It finds particular application in conjunction with magnetic resonance image reconstruction and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find utility in other signal processing techniques in which undersampled signals are processed by transforms or other mathematical operations that are cyclically repeating, e.g. Fourier transforms.
For rapid data acquisition, such as single shot cardiac imaging, rectangular fields of view are often used to reduce the data acquisition time. More specifically to cardiac imaging, a typical field of view might be defined by the boundaries of the patient's torso. The heart, of course, only occupies a fraction of this field of view. The size of the field of view is related to the amount which the phase encoding gradient is stepped, incremented or decremented, between adjacent views. By appropriately selecting the phase encode gradient steps, data generation and collection can be limited to the half, quarter, or other fraction of the whole torso field of view, i.e. undersampled.
A Fourier transform operation tends to treat image data as if the subject were an infinite series of identical subjects at regular spacing. Structure bordering a selected field of view is transposed into the selected field of view and superimposed on the resultant image by the Fourier transform operation. That is, the Fourier transform "assumes" that the selected field of view and the adjoining fields of view contain identical samples and superimposes the images. This superimposed out-of-field-of-view structure is denoted herein as roll-over aliasing. In the cardiac imaging example in which only the central patient portion with the heart is imaged, patient data from the portion of the patient above the undersampled data and from below the portion of the patient corresponding to the undersampled data becomes superimposed on the resultant image as artifacts. Various techniques have been utilized for suppressing these artifacts.
In presaturation, RF pulses are applied prior to the main imaging sequence to saturate spins in the image slice that are located outside the desired region of interest. In the above cardiac example, the presaturation pulses saturate the spins of tissue above and below the central portion of the patient in which the heart is located corresponding to the undersampled region. The imaging procedure is performed immediately following presaturation while the spins are still saturated. Because the spins in the presaturated region have only a very short time to recover longitudinal magnetization, they contribute very little signal to the resultant image data set, reducing the alias signal.
In selective volume techniques, two or more radio frequency pulses are applied to excite and refocus the magnetization in only a preselected region of interest. In an exemplary embodiment, 90.degree. and 180.degree. radio frequency pulses are applied with slice select gradients on each of two axes in a spin echo sequence. Only the material in the volume seeing both RF pulses, i.e. the intersection of the two orthogonally defined regions, generates a signal with the appropriate phase history to be refocused during data collection.
In the selective undersampling technique, the region of interest is centered on the zero phase encode axis. The zero phase encode view has the most energy of all of the views with the highest phase encode view having the least. By having the region of interest correspond to the central views, the views with the most energy per view are imaged and the relatively weak views are not. Because most of the energy or signal magnitude is located at the central views of the raw data set, most of the energy does not alias--it is fully sampled. Intrinsically, the unsampled, high frequency information does alias. There is no suppression of this high spatial frequency information. However, the high spatial frequency information is of a sufficiently lower energy level or signal magnitude that the aliasing which it contributes to the resultant image is relatively weak and hard to discern.
These three techniques each have disadvantages. The selective volume technique is limited to RF spin echo sequences. Such spin echo sequences require two RF pulses as opposed to the single RF pulse used for field echoes. Thus, spin echoes are inherently less well suited to high speed imaging than field echoes. Further, they tend to require much higher specific absorption rates than field echoes. The selective volume techniques are also subject to imperfect slice profiles along the phase encoding/slice select direction. This results in image non-uniformity. A compensation is made by phase encoding for larger than the ideal field of view which results in a less than optimal reduction in the minimum number of phase encoding views.
The presaturation pulse techniques are subject to imperfect slice profiles which gave rise to non-uniformities in the undersampled regions. They are also subject to imperfect radio frequency calibration which gives rise to incomplete suppression of aliased information. When only a single preparatory presaturation pulse is applied prior to the acquisition of the entire raw data set, there may be a significant recovery of longitudinal magnetization causing significant levels of aliasing. If more than one presaturation pulse is used, e.g. one pulse before each phase encoding view, the gradient demands and specific absorption rates are vastly increased. The presaturation pulses are subject to imperfect calibration of radio frequency pulses which can result in inadequate alias suppression. Moreover, the side lobes of the presaturation pulses can themselves produce artifacts and non-uniformities within the image region of interest.
Selective undersampling makes no attempt to prevent the aliasing of high spatial frequency information. The lower energy levels at the high spatial frequency information in the central views tend to make the artifacts more subtle and difficult to distinguish from genuine structure creating more uncertainty as to the diagnostic interpretation of the resultant image. Moreover, the resolution information corresponding to the non-sampled high spatial frequencies is lost.
The present invention contemplates a new and improved technique for suppressing aliasing in undersampled data.